1. Field of the Invention
The present invention relates to an apparatus for applying a coating composition to a support, and more particularly, it relates to an apparatus having an extruder for continuously extruding a coating composition onto a surface of a moving support, to thereby coat the support with a uniformly thick layer of the coating composition.
2. Background of the Invention
The concept of the term "support" used in this specification generally includes a flexible belt like material with a width of 0.3 to 3 m, a length of 45 to 10,000 m and a thickness of 5 to 200 .mu.m. Such a support may be a film of resin such as polyethylene terephthalte, polyethylene-2,6-naphthalate, cellulose diacetate, cellulose triacetate, cellulose acetate propionate, polyvinyl chloride, polyvinylidene chloride, polycarbonate, polyimide, polyamide and the like. It may be paper, or coated or lamination paper of .alpha.-polyolefin having 2 to 10 carbon atoms, such as polyethylene, polypropylene, ethylen-butene copolymer and the like. It may also be a metal foil of such as aluminum, copper, tin and the like. The concept of the term "support" further includes belt-like material provided by applying previously processed layers onto surfaces of the aforementioned belt-like material as substrates.
In general, supports of the type as described above may be used for the step of applying a coating composition, such as a photographic light-sensitive coating composition, a magnetic coating composition, a surface-protecting coating composition, an antistatic coating composition, a lubricating coating composition or the like to the surface of the support according to the desired purpose. The coating composition applied to the support is dried and the support is cut to a necessary width and length to thereby attain manufactured articles. Typical examples of such manufactured articles are various kinds of photographic film, photographic paper, magnetic tape and the like.
Heretofore, in order to apply the coating composition very thinly and uniformly while moving the support at a relatively high speed, for example, at 50 to 100 m/min, extruders using a doctor blade have been studied and surveyed from various approaches. Typical examples of methods thus studied include a web pressure type extrusion method as schematically shown in FIG. 1, a parallel slit extrusion method as shown in FIG. 2, and the like.
The conventional extruder 10 as shown in FIG. 1 has been disclosed in U.S. Pat. No. 4,424,762. In FIG. 1, the extruder 10 has a top end portion formed by a doctor blade 11 and a front blade 12. The term "front blade" herein used means a blade disposed in opposition to the doctor blade. The blades 11 and 12 form an exit area where the coating composition is extruded from the slot onto a support W. The support W is pressed onto the protruding surfaces of the blades 11 and 12 by guiding means such as guide rollers and the like so as to be slightly curved along the surfaces of the blades 11 and 12. Then, the distance between the support W and the doctor blade 11 located at the back of the slot with respect to the direction of movement of the support W is changed corresponding to the change in supply quantity or extrusion quantity to the coating composition extruded from the slot, thereby applying the coating composition to the surface of the support.
However, there exists a problem that a change is caused in the distance between the support W and the doctor edge 11 by various factors which are dependent on the support W, for example, a change in the thickness of the support W, irregularity in Young's modulus of the support W, and the like. These changes result in irregularity in thickness of the film formed on the support W by applying the coating composition thereto. There exists a further problem that foreign, substances such as dust particles and the like which have been carried on the support W are often caught by the doctor blade 11, so that streak-like irregularity of film occurs along the direction of movement of the support.
The conventional extruder 20 as shown in FIG. 2 has been disclosed in U.S. patent application, Ser. No. 824,193, filed on August 12, 1977. The extruder 20 shown in FIG. 2 is different from that shown in FIG. 1 in that the extruder 20 has no doctor blade. The extruder 20 is free from the aforementioned problem of streak-like irregularity caused by catching foreign substances. Particularly, in the case where a high viscosity coating composition is applied at high speed, a large discharge rate is required at the coating composition discharge side of the slot so that the slot gap (l) must be small. Hence, pressure loss of the coating composition which passes through the slot becomes a problem. In other words, pressure at the coating composition discharge side is too low relative to the pressure before the coating composition enters into the slot gap. Accordingly, the coating composition must be supplied to the extruder with very high pressure, for example, 50 kgf/cm.sup.2 (pressure differs according to the conditions of application). Accordingly, a high pressure metering pump must be provided. However, there exists a serious problem in that it is difficult to produce such a high pressure metering pump and such a pump is expensive even if it can be produced.